1. Field of the Invention
The present invention relates to a developer bearer, a developing device having the developer bearer, and an image forming apparatus having the developing device.
2. Discussion of the Background Art
Conventionally, a developing device including a developer bearer having plural electrodes, to which different voltages are applied, is well known. For example, there exists a developing device that develops a latent image formed on an image bearer, such as a photoconductive member, etc., by supplying developer carried on a developer bearer distanced therefrom. The developing device sometimes employs a system of making one component developer (toner) into a cloud state and supplying it onto a latent image bearer. A developer bearing member employed in the system includes plural kinds of electrodes arranged along an outer circumferential surface thereof at a prescribed pitch and a protection layer overlying such electrodes. When electric fields are created between plural kinds of electrodes neighboring to each other and timely changed by applying different voltages each changing as time elapses to the plural kinds of electrodes, toner on the developer bearing member can soar therebetween due to the electric fields. Such phenomenon is called flare. Thus, the toner becomes the cloud state in the vicinity of the outer circumferential surface of the developer bearing member.
In this system of the developing device, to create the flare while avoiding the toner from attracting to the outer circumferential surface of the developer bearing member, a power relation between a force F1 applied to the toner from a flare use electric field created between the plural kinds of electrodes neighboring to each other and an attraction force F2 attracting the toner to the outer circumferential surface of the developer bearing member becomes important. Specifically, when the F2 is larger than the F1, the toner cannot escape from the attraction force directing to the outer circumferential surface of the developer bearing member, and thus, does not create the flare. Whereas when the F1 is larger than the F2, the toner can create the stable flare in proportion to the difference between these F1 and F2. When the F1 is increased, the above-mentioned difference can also increase, thereby the stable flare can be obtained. To increase the F1, a flare use electric field created on the outer circumferential surface of the developer bearing member is necessarily increased.
The Japanese Patent Application Laid Open No. 2007-133388 discloses a developing device having two kinds of electrodes arranged on the same circumference of a roller state developer bearing member to create a flare electric field. These electrodes are formed in a comb teeth state meshing with each other while being arranged along the outer circumferential surface of the developer bearing member. Then, by applying the above-mentioned voltages to the respective kinds of electrodes, the toner can soar and create the flare between the comb teeth sections.
The Japanese Patent Application Laid Open No. 2008-116599 discloses a roller type developer bearing member having three kinds of electrodes for creating the flare use electric field. Among the three kinds of electrodes, two of those are arranged on the same circumference. The remaining one is arranged on the outer circumferential surface side of the two kinds of electrodes. By applying three different phase voltages to respective electrodes, toner can soar due to the flare created between the respective electrodes.
Further, a developing device described in the Japanese Patent Application Publication No. 1-31611 includes two kinds of electrodes arranged on a developer bearing member for creating an alternating current electric field (charging use electric field) that vibrates and charges toner on the developer bearing member. These two kinds of electrodes are insulated by air provided therebetween. However, a space between these electrodes is not covered with insulation material.
To increase the flare use and charge use electric fields using the developing device in which the plural kinds of electrodes are arranged on the same roller circumference as mentioned in the Japanese Patent Application Laid Open Nos. 2007-133388 and 2008-116599 and Japanese Patent Application Publication No. 1-31611 as above, leakage necessarily occurs and is to be prevented. Since either air or insulation member, such as resin, etc., is filled up between the electrodes, insulation can sufficiently be achieved in deed when relatively small voltages are applied to create an electric field. However, when relatively large voltages are applied to create a larger electric field, the insulation can hardly be sufficiently achieved for the reasons as mentioned below.
In the Japanese Patent Application Laid Open No. 2007-133388, metal plating is applied to the surface of plastic roller having comb teeth sate grooves, and the surface is shaved, thereby two kinds of comb teeth sate electrodes are produced. As another manner of producing such two kinds of comb teeth sate electrodes, the surface of a metal plated roller is etched. Otherwise, conductive ink is ejected using an ink jet system for the same purpose. However, in either case, the insulation between the two kinds of electrodes is achieved by coating and filling up the roller surface with insulation material having comb state electrodes. Thus, a boundary face is formed between two kinds of right and left electrodes and between the lower side resin surface of the roller and the upper side coated insulation material. Thus, leakage likely occurs through the boundary face, and accordingly, the insulation between the electrodes is hardly maintained sufficiently when relatively a large voltage is applied.
Further, as described in the Japanese Patent Application Laid Open No. 2008-116599, among the three kinds of electrodes, two kinds of those are arranged on the same circumference, and two kinds of electrodes are similarly produced as in the Japanese Patent Application Laid Open No. 2007-133388. Accordingly, the leakage likely occurs via a boundary face. Thus, for the same reason as in the Japanese Patent Application Laid Open No. 2008-116599, the insulation between these two electrodes is hardly maintained sufficiently. However, since the insulation layer is arranged between the remaining one kind of the electrode and the two kinds of electrodes among the three, the leakage does not occur therebetween. However, when the leakage occurs between the two kinds of electrodes, an appropriate flare use electric field cannot be created.
Further, in the Japanese Patent Application publication No. 1-31611, since insulation material does not cover a gap between the two kinds of electrodes, leakage necessarily occurs via toner when the toner is filled up between the electrodes.
The above-mentioned leakage necessarily occurs regardless of voltage application in a developer bearing member as far as that includes plural kinds of electrode members which simultaneously receive different voltages from each other.